Instead of steel wire ropes that have been used successfully on cranes for many years, it is recently being tried to use high-strength fiber ropes made of synthetic fibers such as aramid fibers (HMPA), aramid/carbon composites, highly modular polyethylene fibers (HMPE) or poly(p-phenylene.2,6-benzobisoxazole) fibers (PBO). The advantage of such high-strength fiber ropes is their low weight. At equal rope diameters and equal or higher tensile strengths, such high-strength fiber ropes are clearly lighter in weight than comparable steel wire ropes. In particular for high cranes with accordingly long rope lengths, this results in a major weight reduction reflected in the dead weight of the crane leading to higher payloads for an otherwise unchanged crane design.
However, a disadvantage of such high-strength fiber ropes is their breaking behavior, i.e. their failure without any distinct long-term prior warning. While wear is clearly indicated with steel wire ropes, showing failure long in advance, for example when individual steel wires break and splice open, which is easily detected, high-strength fibers show few signs of excess splicing that could be detected with the naked eye and that would show long before their actual failure. They therefore require intelligent monitoring measures to allow the early detection of when the discard state of high-strength fiber ropes will occur.
It is known from WO 2012/100938 A1 to detect the discard state of a high-strength fiber rope by testing various rope discard criteria which change over the time in which a rope is used and under stress. Here, the rope diameter, the shear stress stiffness measured by the cross-sectional changes resulting when the rope is pinched, and by the number of completed stress cycles. However, the informative value of these individual discard criteria is limited, which means that the interaction of these discard criteria must be monitored and evaluated in a rather complex monitoring process before the discard state can actually be detected with reliability.
Based on this, it is the object of the present invention to provide an improved device for detecting the discard state of high-strength fiber ropes which avoids the disadvantages of the prior art and advantageously develops it further. Preferably, a simple but reliable and precise detection of the discard state is to be achieved which economically utilizes the remaining service life if the fiber rope without jeopardizing safety, and which can be used on construction machinery with simple detection means functioning reliably even under heavy-duty working conditions.
According to the invention, the above object is achieved with a device according to the present application.
It is therefore suggested to monitor the rope's torsional stiffness and to determine the discard state by means of the rope's torsional stiffness. The rope's torsional stiffness means the rope's resistance or moment of resistance against the rope being twisted. It takes a certain torque to twist the end sections of a rope sector lengthwise relative to each other, i.e. to chordate or twist the rope, to achieve the above twist whereby the said torque depends on the rope's stiffness. According to the invention, the detection means comprises torsional stiffness determination means for determining the rope's torsional stiffness, whereby the evaluation unit provides the discard signal depending on the rope's determined torsional stiffness. While steel wire ropes do not show significant changes in torsional stiffness depending on the rope's service life, this is different with high-strength fiber ropes. The filaments which are still flexible at the beginning of the rope's use, are made harder and the rope is made stiffer by the tensile stress and the bending stress. This increase in the rope's torsional stiffness is easy to measure, which means that the discard state can be determined reliably and precisely by the rope's monitored torsional stiffness. It shows that rope twisting tests with a new rope show a rather low torsional stiffness while ropes driven to the breaking point show a very high torsional stiffness in the end due to prolonged and severe stress, namely many times that of the rope's original state. This increase rises continuously with the cycles-to-failure rate, reaching the highest point when the rope breaks, which means that the evaluation unit can determine the discard state relatively easily.
In the further development of the invention, the torsional stiffness determination means can comprise a swivel that can be integrated in the rope drive or used to sling the rope. Sometimes such a swivel is also called the rope swivel; it usually consists of two swivel parts which can be twisted relative to each other in the rope's lengthwise direction, especially about a rotational axis coaxially to the rope's longitudinal axis, whereby for example a fixed swivel part can be non-rotatably secured on a crane boom in longitudinal direction, while the rotatable swivel part is non-rotatably connected to the rope. To use such a prior-art swivel to determine the rope's torsional stiffness, a further development of the invention can provide a rotary drive for forcing the two swivel parts to rotate relative to each other, in particular this rotary drive can be integrated in an interior space of the swivel whereby the rotary drive can, for example, be an electric motor, if need be in conjunction with a gear.
The said rotary drive functionally sits between the two swivel parts and is rotationally supported in relation to the two swivel parts, i.e. in relation to a twist about the swivel axis. In particular, a drive axis of the rotary drive, for example a gear output shaft, can be connected with the rotatable swivel part on which the rope is non-rotatably secured, while a motor housing is non-rotatably connected with the non-rotatable swivel part or at least with only limited rotatability.
In a further development of the invention, the swivel can be provided with a torque meter and/or twist angle meter or torsion angle meter to determine the torque applied when the swivel is forced to rotate or to record or determine the twist angle of the two swivel parts relative to each other when the swivel is forced to rotate.
In principle, the said torque meters or torque angle meters can be of various designs. For example, in a further development of the invention, the torque meter can be integrated in the rotary drive, for example when an electric motor is used, it can record its electrical control variables such as currency and voltage to determine the thus generated torque. As an alternative or in addition to such a drive parameter meter, the torque meter can also determine the reaction moment induced by the fixed swivel part in its holding member, for example in the form of a crane boom. As an alternative or in addition to that, the torque meter can also be installed in the drive gear or the rotatable swivel part, for example between the drive gear and the rotatable swivel part, for example in the form of a torsion measurement socket or such.
In principle, the torsion angle meter or twist meter can also be of various designs, for example integrated into the drive gear or its drive motor. As an alternative or in addition, the twist meter can directly record the twist of the two swivel parts relative to each other.
The rope's torsional stiffness can be detected by the torsional stiffness detection means by means of a twist of the rope achieved by a predetermined torque and/r by means of the torque required for a predetermined twist. In a further development of the invention, these two detection criteria can also be used in combination with each other, in particular such that it is determined which force is required for a predetermined twist and which twist occurs with a predetermined torque, to take into consideration the non-linearity that may result with torsional stiffness.
To avoid distorting or influencing the measuring of the rope's torsional stiffness by the stress of tensile forces upon the rope, the torsional stiffness determination means comprises a tensile stress adjuster which sets the same tensile force conditions on the rope for the repeating detection of the rope's torsional stiffness. In particular, the said tensile stress adjuster can comprise a tensile stress release means which essentially completely releases the rope of tensile forces when the rope's torsional stiffness is determined.
The said tensile release means can be of various designs. In an advantageous embodiment of the invention, the tensile release means can comprise holding means to hold the rope in lengthwise direction, preferably at least one rope clamp for clamping the rope, in particular for catching hoist loads on the lifting hook and the rope section to be tested for the rope's torsional stiffness, whereby the said holding means can for example be provided on the trolley of a tower crane, to relieve the rope section between the trolley and the swivel. Preferably, the test of the rope's torsional stiffness test can be conducted without a load on the lifting hook, whereby the lifting hook is preferably moved to a predetermined height by control device or manually to achieve a predetermined tensile force of the rope through the rope's dead weight and to have a certain rope length for testing.
In an advantageous further development of the invention, the torsional stiffness determination means comprises a swivel compensator which prior to conducting the test of the rope's torsional stiffness eliminates or at least greatly reduces any twist that might be present on the rope. Normally, ropes twist which are coiled on rope drums or run around rope pulleys. To avoid the distortion of the rope's torsional stiffness measurements, preferably such twist can be removed prior to the torsional stiffness test being conducted. Preferably, the said swivel compensator can also be integrated in or associated with the above described swivel. For example, the swivel compensator can comprise a rotational direction sensor which helps to determine the rotational direction or effective direction of the rope's twist, which means that the rotary drive can be activated depending on an associated rotational direction signal to turn the rope via the swivel or its rotatable swivel part in the intended rotational direction. If need be, the height of the torque at the swivel induced by the rope's twist can be determined by a torque sensor of the above described kind to activate the rotary drive for as long as the detected or determined torque induced by the rope's twist approaches zero before the torsional stiffness test is begun.
In principle, the evaluation arrangement for providing a discard signal can work in various ways, for example by monitoring changes occurring in the rope's torsional stiffness and/or by monitoring absolute values of the torsional stiffness. In particular, the said evaluation unit can be designed such that a discard signal is provided when the rope's torsional stiffness and/or its change reaches an associated threshold value.
For example, one or more reference measurements can be conducted with a new rope such that the percentage change in a rope's torsional stiffness occurring in operation can be compared with a threshold value for change, and if this value is exceeded or approached, the discard signal is provided. In particular, the discard signal can be provided when the rope's torsional stiffness increases beyond a still tolerable threshold value. As an alternative or in addition, the monitored and in operation constantly or cyclically detected torsional stiffness of a rope can be compared with an absolute threshold value predetermined by the manufacturer for a certain type of rope or a specific rope, and to provide the discard signal when this threshold value is reached or exceeded. Also as an alternative or in addition, the discard signal can be provided when the change in a rope's torsional stiffness as determined by measuring occurs to fast and/or too slowly, i.e. when the speed of change in the rope's torsional stiffness over a given time exceeds or remains below a threshold value. The speed of change over a given time can be the speed of change over the number of stress cycles recorded for example with a load cycle counter which can be taken into account by the evaluation arrangement. As an alternative or in addition, the speed of change can also only be taken into account by means of the number of measurements of the rope's torsional stiffness, for example such that discard signal is provided when the change in the rope's torsional stiffness detected after a certain number of measurements, for example after the tenth measurement, exceeds a certain predetermined threshold value.
The discard signal can simply be indicated to the crane operator, for example acoustically and/or visually, or it can be used to stop the rope drive.
In an advantageous further development of the invention, the torsional stiffness determination means can be firmly installed in the rope drive of the hoist such that the rope's torsional stiffness can be constantly monitored during operation, i.e. in the operational state of the hoist, without the necessity of having to convert the hoist into a special test modus. As an alternative or in addition, the torsional stiffness determination means can also be provided as a detachable unit that can be used in different hoists.
Below, the invention is described in more detail by means of a preferred embodiment and with reference to the drawings.